1. Field of the Invention
The present invention relates to an improved antenna system for automobiles, which can efficiently detect waves received by the vehicle body and transmit the detected signals to various built-in receivers.
2. Description of the Prior Art
Antenna systems are essential for modern automobiles to positively receive various waves such as radio waves TV waves, car-telephone waves and other waves at their built-in receivers. Antenna systems also are very important for citizen band transceivers effecting the reception and transmission of waves between the automobiles and other stations.
There is generally known a pole type antenna which projects outwardly from the vehicle body and exhibits favorable reception performance. However, the pole type antenna always constituted an obstruction in vehicle body design.
The pole type antenna is actually subject to being damaged or stolen and also produces an unpleasant noise when an automobile, on which that antenna is mounted, runs at high speeds. For these reasons, it has been desired to eliminate the pole antenna.
Recently, frequency bands of broadcast and communications waves to be received by built-in receivers in automobiles have been widened. This requires a plurality of antennas mounted on an automobile to be compatible with various frequency bands. The plurality of antennas will detract from aesthetic automobile design and further provides a problem in that electrical interference between the antennas will degrade the reception performance in the antennas themselves.
Some attempts have been made to eliminate or conceal the pole type antenna. One of these attempts is that an antenna wire is applied to the rear window glass on an automobile.
Another attempt is an antenna system which is adapted to detect surface currents induced on the body of an automobile, by radio or other waves. This attempt appears to be the most positive and efficient. However, experiments always showed that such an antenna system could not advantageously be utilized.
One of the reasons why the prior art antenna system cannot effectively utilize the surface currents induced on the vehicle body by radio or other waves is that the level of the surface currents is not as would high as be expected. The prior art utilizes surface currents induced on the vehicle body at its front roof. Notwithstanding, one cannot detect surface currents at such a level, so as to be effectively utilized.
The second reason is that surface currents contain a very high proportion of noise which is due to the ignition and regulator systems of an engine. Therefore, the noise cannot be eliminated unless the engine does not operate.
One proposal to overcome the above problems is disclosed in Japanese Patent Publication No. Sho 53-22418 in which an electrically insulating member is formed on the vehicle body at such a location that surface currents flow concentrically. Surface currents are sensed directly by a sensor between the opposite ends of the insulating member. Although such an arrangement can practicably detect signals having an S/N ratio, it requires a pickup which must be mounted in a notch formed particularly in the vehicle body. This is not acceptable for mass-production.
Another proposal is disclosed in Japanese Utility Model Publication No. Sho 53-34826 in which an antenna system includes a pickup coil for detecting surface currents flowing on a pillar of the vehicle body. Although this proposal is advantageous in that the antenna system can be fully mounted within the vehicle body, it is not practical since the pickup coil must be located near the pillar in a direction perpendicular to the length thereof. Furthermore, it appears that such a pickup arrangement is not practical, since it cannot obtain antenna outputs at any practicable level.
It will be apparent from the foregoing that the prior art did not necessarily solve various problems particularly with respect to the pickup arrangement for efficiently detecting surface currents induced on the vehicle body by radio or other waves and also for obtaining a practicable level of S/N ratio. Rather, experiments showed that the antenna system utilizing surface currents on the vehicle body cannot probably be used in principle.
Moreover, the prior art pickup device is poor in directional pattern and yet tends to produce multi-path noise when it is used to receive FM radio waves belonging to VHF frequency bands since these waves are interferingly reflected by buildings and mountains.